Render sort order should consider shared shaders to minimize switching

[brianchirls]

The painterSortStable function sorts objects to be rendered by material to minimize switching between materials. However, it doesn't consider that now different materials can share a common shader program. The result is that the renderer may be switching out shader programs more often than necessary.

I put together a rough benchmark to test this patch. Here is one using the existing code:
http://output.jsbin.com/tuneso

And here is one using the patched code:
http://output.jsbin.com/nogovi

In Chrome on my Late 2013 MacBook Pro, with 10,000 objects, I'm seeing ~4 fps unpatched and ~13 fps patched. On my new PC w/ GeForce 980, I get ~26 fps vs ~38 fps. Please adjust the object count on your system accordingly.

This is related to #8849.

[bhouston]

Sweet. This doesn't affect the draw order of the transparent objects does it?

[mrdoob]

Sweet. This doesn't affect the draw order of the transparent objects does it?

It doesn't. Those are sorted using reversePainterSortStable().

[mrdoob]

@tschw looks good?

[mrdoob]

Hmm, I've just checked WebGLProgram and I think this shouldn't have side effects.

[mrdoob]

Many thanks!

[brianchirls]

@mrdoob Thanks for the lightning-fast review and merge!